Tailoring the characteristics of gating transitions in the porous network, Ni(ndc)dabco (ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane), also termed DUT-8(Ni) (DUT = Dresden University of Technology), was achieved by systematically adjusting the critical synthesis parameters. The impact of the starting composition and solvent mixtures in the synthesis was found to critically affect the guest-response properties of the obtained materials. A comprehensive set of physical characterization methods, namely thermal analysis, H NMR of digested crystals, solid stateC NMR, PXRD, SEM, IR and Raman spectroscopy shows that the crystallite size is a crucial factor, determining the differing characteristics such as "gate pressure" and adsorption capacity in the guest-responsive switching behaviour of DUT-8. Crystallites smaller than 500 nm in size retain the open form after removal of the guest molecules resulting in typical "Type Ia" isotherm, whereas crystallites larger than 1 μm transform into the "closed pore" form and therefore can show a characteristic "gate opening" behaviour during gas adsorption. The particle size distribution of DUT-8(Ni) can be tailored by changing the synthesis conditions and consequently the slope of the isotherm at the "gating step" is affected. The in depth analysis of synthesis conditions and switching behaviour is an important step towards a better understanding of the fundamental principles responsible for guest responsive porosity switching in the solid state.

The adsorption/desorption cyclability of four flexible MOFs, namely, MIL-53(Al), ELM-11, DUT-8(Ni), and SNU-9, was studied at 298 K using n-butane as adsorptive. The detailed analysis of thermal response curves, physisorption isotherm data, powder X-ray diffraction patterns, as well as SEM images revealed the highly stable switching performance of MIL-53(Al) and ELM-11 materials during 100 adsorption/desorption cycles. In contrast, for DUT-8(Ni) and SNU-9, the multiple adsorption/desorption stress leads to the reduction of crystallite size, causing changes in the switching behavior in the initial 10 physisorption runs, and a characteristic shift of the "gate-opening" pressure to higher values is observed.

The metal-organic framework (MOF) DUT-8(Ni) (DUT = Dresden University of Technology) shows a structural transformation from a non-porous to a porous phase during the adsorption of gases. A rigid derivative of this material has recently been synthesized, where this "gate pressure like" flexibility is completely absent. This rigid derivative of DUT-8(Ni) always stays in the porous phase even in the absence of any adsorbate. This motivates the present investigation of the adsorption of nitric oxide (NO) on the flexible and rigid forms of DUT-8(Ni) by continuous wave electron paramagnetic resonance (EPR) spectroscopy at Xband frequency. The EPR signal of desorbed NO is measured at moderate temperatures and the decrease of its intensity indicates the adsorption of this gas within the porous phase of DUT-8(Ni) at low temperatures. An adsorption and desorption related hysteresis loop of the intensity of this signal is observed for the flexible but not for the rigid . This difference might reflect the difference in the flexibility of both materials. Furthermore EPR signals with electron spin S = 1/2 are measured, which can likely be attributed to Ni 2+ -NO adsorption complexes at defective paddle wheel units within the porous phase of with the unpaired electron sitting at the Ni 2+ ion. The order of their g-tensor principle values allows a distinct characterisation of the ligand environment of these ions. Defects for which the EPR signals indicate, that at least one NDC (2,6-naphthalenedicarboxylate) ligand molecule does not coordinate to the paddle wheel, are only observed for the rigid but not for the flexible . Also the density of defective paddle wheel units with only one Ni 2+ ion or a missing dabco (1,4 -diazabicyclo[2.2.2]octane) ligand is indicated to be one order of magnitude larger in the rigid than in the flexible derivative of this MOF. The observed differences in the presence and amount of distinct defects might be related to the difference in the flexibility of both forms of the investigated material.

Metal–organic frameworks are promising candidates for selective separation processes such as CO2 removal from methane (natural gas sweetening). Framework flexibility, that is, the ability of a MOF lattice to change its structure as a function of parameters like pressure, temperature, and type of adsorbed molecules, is only observed for some special compounds. The main question of our present work is: does framework flexibility influence the adsorption selectivity? As a direct quantitative method to monitor the adsorption of both, carbon dioxide and methane, we make use of high-pressure in situ 13C NMR spectroscopy of 13CO2/13CH4 gas mixtures. This method allows to distinguish between the two gases as well as between adsorbed molecules and the interparticle gas phase. Gas mixture adsorption is studied under isothermal conditions. The selectivity factor for CO2 adsorption from CO2/CH4 mixtures is measured as a function of total gas pressure. The flexible material SNU-9 as well as the flexible and the nonflexible variant of DUT-8­(Ni) are compared. Maximum selectivity factors for CO2 are observed for the flexible variant of DUT-8­(Ni) in its open, large-pore state. In contrast, the rigid variant of DUT-8­(Ni) and SNU-9 especially in its intermediate state exhibits lower adsorption selectivity factors. This observation indicates significant influence of the framework elasticity on the adsorption selectivity.

Variation of the crystallite size in flexible porous coordination polymers can significantly influence or even drastically change the flexibility characteristics. The impact of crystal morphology, however, on the dynamic properties of flexible metal-organic frameworks (MOFs) is poorly investigated so far. In the present work, we systematically modulated the particle size of a model gate pressure MOF (DUT-8(Ni), Ni2(2,6-ndc)2(dabco), 2,6-ndc−2,6-naphthalenedicarboxylate, dabco−1,4-diazabicyclo[2.2.2]octane) and investigated the influence of the aspect ratio, length, and width of anisotropically shaped crystals on the gate opening characteristics. DUT-8 is a member of the pillared-layer MOF family, showing reversible structural transition, i.e., upon nitrogen physisorption at 77 K. The framework crystalizes as rod-like shaped crystals in conventional synthesis. To understand which particular crystal surfaces dominate the phenomena observed, crystals similar in size and differing in morphology were involved in a systematic study. The analysis of the data shows that the width of the rods (corresponding to the crystallographic directions along the layer) represents a critical parameter governing the dynamic properties upon adsorption of nitrogen at 77 K. This observation is related to the anisotropy of the channel-like pore system and the nucleation mechanism of the solid-solid phase transition triggered by gas adsorption.